This invention relates to a fuel supply control method for internal combustion engines, and more particularly to a method of this kind which is adapted to supply the engine with a quantity of fuel optimal to operating conditions of the engine, after termination of a fuel cut action which is effected at deceleration of the engine, thereby improving the emission characteristics, driveability and fuel consumption of the engine.
In conventional methods of controlling the fuel supply to an internal combustion engine in response to operating conditions of the engine, by means of an electronically controlled fuel injection system, it is known to cut off the fuel supply to the engine at deceleration for improvement of the fuel consumption and emission characteristics of the engine, and then increase the fuel quantity being supplied to the engine after termination of the fuel cut operation, so as to improve the driveability of the engine. As such after-fuel cut fuel control methods, it has been proposed by Japanese Utility Model Provisional Publication (Kokai) No. 53-33721 to increase the fuel quantity by setting a longer fuel injection period for a predetermined period of time starting from the termination of a fuel cut operation, and it has also been proposed by Japanese Patent Provisional Publication (Kokai) No. 56-47631 to increase the fuel quantity by an amount corresponding to the duration of the immediately preceding fuel cut operation.
According to the above proposed methods, the increase of the fuel quantity after the termination of a fuel cut operation is based upon the fact that if the clutch is disengaged immediately after the termination of the fuel cut operation, the engine speed suddenly drops to cause engine stall, and to avoid such engine stall, it is neccessary to supply a sufficient amount of fuel to the engine immediately after the termination of a fuel cut operation.
However, if the clutch is kept in its engaged position after the termination of a fuel cut operation, the engine is free from a sudden drop in the rotational speed, and therefore there is no fear of occurrence of engine stall. If nevertheless the fuel quantity is increased on such occasion, the air/fuel ratio will become overrich, resulting in deterioration of the emission characteristics, increased fuel consumption, etc. Besides, after the termination of a fuel cut operation, the air/fuel ratio of the mixture being supplied to the engine is prone to become overrich by the following reason, too. That is, as generally known, the actual intake air quantity supplied to the engine during normal combustion operation is larger than that during non-combustion operation (hereinafter merely called "motoring"), or in other words, the charging efficiency of the engine during normal combustion operation is higher than that during motoring. This means that the intake pipe absolute pressure during motoring of the engine is higher than that during normal combustion operation of same. Therefore, if a fuel supply control method in which the fuel quantity is determined at least in dependence on the intake pipe absolute pressure is applied after the termination of a fuel cut operation, an excessive quantity of fuel is supplied to the engine immediately after a transition from the fuel cut operation or motoring to a normal combustion operation, resulting in increased fuel consumption, deteriorated emission characteristics and degraded driveability of the engine, due to the phenomenon that the intake pipe absolute pressure during fuel cut operation or motoring is higher than that during normal combustion operation.